While 1921 dated police rework Lugers are scarcer, they are apparently not so scarce as to raise their value much.
The refinishing work may have been done when it was reworked for police usage. You can only determine this by judging the state of the finish to estimate it's age. If you can see red oxide deeply in the finish using a magnifying glass and incandescent or natural light, the finish may, in fact, be from the 1920s.
Early Erfurt Lugers (the armory was active from 1910 to 1918) were manufactured without stock lugs. If this one doesn't show evidence of removal, it could be an early Erfurt. What else make you think it is an early one? What is the serial number? Here's a link from Jan's site:
Discussion of 1912 Erfurt Lugers
Marc
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Igitur si vis pacem, para bellum -
- Therefore if you want peace, prepare for war.